Chain/drive skipping/lurching

I know the usual reasons why a chain/drive lurches/skips under pressure, but my bike is doing this mightily, and I cannot figure it out at all.
Here's the timeline of what I've done so far:
It seems/ed to be lurching more on the higher gears, but it does it on pretty much all 9.
I put a new chain on (not for this issue) and the problem got drastically worth. So that seem to point to it being wear issues making a mismatch between chain and something.
I have a normal 9 speed shimano cassette, which seem in pretty good shape to me, so I haven't swapped that out yet to test.
I DID realize the little idler wheel in my chainline (power side is toothed) was worn down to almost nothing, so I swapped that out.
But that didn't help at all. I thought that had to be the problem…I could physically see the chain just swimming over the tooth nubs.
I replaced my jockey wheels, made sure the derailleur is in line (not jumping gears), checked that my pawls seem ok, and checked the front ring teeth. All that seems good.
The jumps are so fast and hard, and because it's really hard to pedal and look with this looong bike on the stand, that I can't really tell where exactly the chain is catching, jumping, whatever.
As far as I CAN see, it seems to be vertically lifting up and off the idler's teeth and skipping teeth. But they are brand new, full depth, and the chain is new so this makes no sense.
I was about to leave on a tour, and now I can't go until I resolve this.
Does anyone have any idea what might be going on? I'm utterly baffled.
Tomorrow I'll put a new XT derailleur and a new cassette on to totally eliminate that as a possibility.
Forgot, this happened on two different wheels, so I highly doubt it's the cassette pawls or teeth. Both are in good shape.
Cheerio
Photo is pretty much same bike as mine.